The field of clinical chemistry is concerned with the detection and quantitation of various substances in body material, typically body fluids such as blood, urine or saliva. In one important aspect of this field, the concentration of naturally occurring substances, such as cholesterol or glucose, in an individual's blood is determined. One of the most frequently used analytical devices in clinical chemistry for determining the concentration of an analyte in a fluid sample is the test sensor. Upon contacting the test sensor with the fluid sample, certain reagents incorporated into the sensor react with the analyte whose concentration is being sought to provide a detectable signal. The signal may be a change in color as in the case of a calorimetric sensor or a change in current or potential as in the case of an electrochemical system.
For a particular class of electrochemical sensors, e.g., amperometric sensors, the detected current is proportional to the concentration of the analyte in the fluid sample being tested. Those systems which employ an enzyme in the reagent system may be referred to as biosensors since they rely on the interaction of the enzyme (a biological material) with the analyte to provide the detectable response. This response, whether it be a change in color or in current or in potential, is typically measured by a meter, into which the sensor is inserted. The meter typically provides a readout of the analyte concentration such as by means of a LCD system.
Glucose is one such analyte that is of particular importance. The determination of glucose in blood is of great importance to diabetic individuals who must frequently check the level of glucose in connection with regulating the glucose intake in their diets and their medications. While the remainder of the disclosure herein will be directed towards the determination of glucose in blood, it is to be understood that the procedure and apparatus of this invention can be used for the determination of other analytes in other body fluids or even non-fluid body materials such as the detection of concealed blood in fecal material upon selection of the appropriate enzyme. In addition such sensors can be used in, for example, testing for meat spoilage or foreign substances in well water.
Diagnostic systems, such as blood glucose measuring systems, typically calculate the actual glucose value based on a measured output and the known reactivity of the reagent sensing element used to perform the test. The latter information can be given to the user in several forms including a number or character that they enter into the instrument. One method included the use of an element that is similar to a test sensor but which is capable of being recognized as a calibration element by the instrument. The test element's information is read by the instrument or a memory element that is plugged into the instrument's microprocessor board for directly reading the test element.
Various arrangements have been used to provide lot calibration information of sensors, for example, to the instrument. A basic method requires the user to enter a code number which the instrument can use to retrieve calibration constants from a lookup table. The transfer of information, for example, may comprise use of a resistor whose resistance value can be measured by the instrument. Further detail concerning use of a resistor is found in U.S. Pat. No. 5,266,179, which is incorporated herein by reference in its entirety. From the resistance value the calibration constants are recovered from the lookup table.
Problems associated with the prior art are discussed and addressed in U.S. Pat. No. 5,856,195 (the '195 patent), which is commonly assigned and incorporated herein by reference in its entirety. The success of sensing meters designed in accordance with the '195 patent has lead to the development of improved sensing meters and improved sensors. For example, existing sensing meters analyze the sample for a predetermined length of time equal to approximately 30 seconds. New improved sensing meters are not restricted to fixed analysis lengths of 30 seconds.
As taught in the '195 patent, the sensing meters should be calibrated to operate appropriately with the sensors to achieve accurate test results. As the new improved sensing meters are being put to use in the field, the former, older, sensing meters will still be used for an unknown period of time. If calibration codes adapted for characteristics of the new improved meters are used in older meters, test results are likely to be inaccurate. Thus, it would be desirable to avoid such inaccurate test results.